One factor affecting the efficiency of an internal combustion engine is the compression ratio at which the engine operates. Compression ratio is a ratio of the expanded to compressed volume of the engine combustion chamber, and is a measure of the degree to which an air/fuel mixture is compressed before ignition. A high compression ratio in a standard Otto cycle engine will result in the piston performing a longer expansion in the power stroke, and consequently more work, in comparison to the same engine running at a lower compression ratio. Compression ratios of gasoline powered automobiles using regular 87 octane gasoline typically range between about 8.5:1 and 10:1.
Compression ratios are limited by spontaneous ignition of the air/fuel mixture at high temperatures, a problem commonly referred to as either engine knock or auto-ignition. Engine knock occurs as a result of disassociation of the air/fuel mixture into more easily combustible fragments when the mixture is exposed to high temperatures for a sufficiently long period of time. The high temperature exposure can result in these fragments initiating an uncontrolled explosion outside the envelope of the normal combustion front during the power stroke of the engine. Engine knock causes audible and potentially damaging pressure waves inside combustion chamber. Knock is a subset of a more general auto-ignition. In this document we refer to auto-ignition as cases where the ignition happens independent of when the spark is fired, as in homogeneous ignition or a burn initiated by a surface ignition prior to the spark event.
Engine knock can be caused or contributed to by a variety of factors in addition to high compression ratios. Other factors include:                the octane rating of gasoline used—low octane gasoline will spontaneously ignite at lower temperatures than high octane gasoline;        hot wall temperatures with high surface-to-volume ratios, which tend to increase the heating of the air/fuel mixture;        localized hot spots, such as around the exhaust valve, which may cause localized heating of the air/fuel mixture and knocking in the area of the hot spots;        fast burn rate—high turbulence promotes good mixing and rapid burning of the fuel, which will reduce the likelihood of spontaneous ignition        high inlet turbulence flow field turbulence also increases the temperature rise in the inlet air/fuel mixture which increases the likelihood of spontaneous ignition.        mixture ratio—increasing the quantity of fuel in the mixture up to stoichiometric increases the energy released and hence the pressure and temperature of the end gas        advanced timing can generate high peak pressures and temperatures.        
Thus, the compression ratio, and consequently engine efficiency, in gasoline engines running the Otto cycle are limited by engine knock. Another factor affecting engine efficiency relates to pumping losses resulting from throttling (reduction) the air/fuel mixture. In the traditional Otto cycle, the air/fuel mixture supplied to the inlet manifold is throttled to run the engine at lower loads. Upon throttling the mixture, a negative pressure differential develops between ambient and the inlet manifold, and less air/fuel mixture is pulled into the combustion chamber from the inlet manifold upon the opening the inlet valve during the inlet stage. This increased pressure differential requires more pumping work to move the air/fuel mixture from the manifold to the combustion chamber. Thus, a traditional Otto cycle provides maximum efficiency when the throttle is completely open, a condition referred to as wide open throttle (WOT). Typical engines running the Otto cycle have lower efficiencies at lower loads, where pumping losses result from throttling.
The use of the Atkinson cycle instead of the standard Otto cycle is one known method of increasing the expansion ratio and efficiency at lower loads. FIGS. 1A and 1B show conventional pressure-volume graphs of ideal Otto and Atkinson cycles, respectively. FIG. 1A shows the stages of the standard Otto cycle: air/fuel inlet stage 50, isentropiccompression stage 52, constant volume combustion stage 54, isentropic expansion stage 56, blowdown 58 and exhaust stage 60. As shown, the piston compresses the mixture during the compression stage 52 to the same degree that it expands during the power stage 56.
By contrast, the Atkinson cycle describes a method of engine operation where the effective air/fuel compression stroke is shortened relative to the power expansion stroke. This may be accomplished for example by keeping the inlet valve closed for a portion of the air/fuel inlet stroke, thus reducing the mass of the air/fuel mixture admitted for the compression stroke. Thus, as shown in FIG. 1B, the air/fuel mixture may be drawn in at stage 62 without a change in pressure until a volume V0 of mixture is admitted. At that point, the inlet stroke may continue with no more mixture being admitted. The mixture is compressed during adiabatic stage 66, the mixture is combusted at constant volume in stage 68, the mixture adiabatically expands in the power stage 70, and the exhaust gas is withdrawn in stage 72. As seen, the expansion stage 70 is increased relative to the compression stage 66. The Atkinson cycle increases efficiencies at lower loads by getting more work out of an expansion stroke for a given compression ratio, but is not able to provide high power densities for high load engine operation.
Attempts have been made to run gasoline engines in the Atkinson cycle at low loads for its engine efficiency and in the Otto cycle at high loads for high power density. Such attempts include employing variable compression ratio (VCR) and variable valve timing (VVT) techniques to the engine. With VCR, the combustion chamber minimum volume is changed slowly compared with the piston motion. VCR by itself provides an engine that is both efficient at part loads and powerful at high loads. VCR can be combined with either a throttle or variable valve timing. Combining variable valve timing and variable compression allows Atkinson at low loads and Otto cycle at high loads.
With VVT, the inlet valve supplying the air/fuel mixture to the combustion chamber may for example be held open for a portion of the compression stroke. This allows a high geometric compression ratio, but as there is a smaller mass of the air/fuel mixture during the compression stroke, the temperature of the mixture is controlled by this lower effective compression ratio to prevent knocking. One example of an engine using VVT is the Prius made by Toyota Motor Corporation of Japan. It has the valve timing set so that the intake closes late at low power giving a 4:1 compression ratio while the expansion ratio is 12:1. At high power it changes the valve timing to get effectively a 9:1 compression ratio with the same 12:1 expansion ratio. However, VVT engines require more moving parts and complexity, and the late closing of the inlet valve negates the some of the benefits gained by avoiding pumping losses.
The majority of fuel consumed over the life of a vehicle is from low-load and idle regions. Because of mechanical friction, heat transfer, throttling and other losses, spark ignition internal combustion engines inherently have peak efficiency at high loads and poor efficiency at low loads. Matching an engine specification to a drive-cycle for purposes of vehicle MPG improvements requires shifting the peak efficiency towards the low loads.
Obtaining peak efficiency at low loads can be achieved, for example, by increasing the compression ratio. However, traditional octane fuel, MBT ignition advance, auto-ignition, and engine knock limit the power density (torque) an engine can achieve for a given compression ratio. It is therefore desired to provide an internal combustion engine which is capable of operating efficiently at low loads to maximize fuel economy, and capable of providing high power densities at high loads while avoiding the problems of the prior art solutions discussed above.